The Bavarian interior minister, Joachim Herrmann, said on Monday that the attacker had declared his support for the leader of Islamic State in a video found on his phone. An initial translation of the Arabic-language video showed the man announcing a “revenge” attack against Germany for “all the Muslims it has killed”.

“I think that after this video there is no doubt that the attack was a terrorist attack with an Islamist background,” he said.

Germany’s Federal Criminal Police Office warned that it had 410 leads on possible terrorists currently in Germany.

On Sunday night, in the Bavarian town of Ansbach, the 27-year-old attacker, identified only as Mohammad D, whose asylum application had failed and was waiting deportation to Bulgaria, killed himself and injured 15 others, four seriously.

Hermann said officers discovered videos with “Salafist content” on storage devices seized at the man’s home, along with petrol, soldering bolts, batteries, chemicals and other material that could be used to make a bomb.

The Isis-linked Amaq news agency said on Monday that the attacker was “a soldier of the Islamic State” who had acted “in response to calls to target nations in the coalition” fighting the terrorist group. It offered no evidence to support the claim.

The attack took place two days after an 18-year-old German-Iranian gunman killed nine people in the Bavarian city of Munich and a week after a man attacked train passengers with an axe in Würzburg, also in Bavaria.

The three incidents have plunged the south-east German state – and the country as a whole – into an acute state of nervousness and prompted difficult questions about the extent to which the open-door policy of the chancellor, Angela Merkel, towards refugees last summer might be to blame.

In another incident on Sunday, a 21-year-old Syrian killed a pregnant Polish woman, 45, with a machete in Reutlingen, in the state of Baden-Württemberg. Police said there were no indications of terrorism and the attacker appeared to have known the woman he killed.

But extremism experts said even though the incident did not appear to have been politically motivated, it would only contribute to the increasingly common perception that refugees were a risk to ordinary Germans’ security.

The German interior minister, Thomas de Maiziere, announced that the country will beef up its police presence at airports and train stations and carry out stop-and-search operations after the four attacks. “What seems particularly important to me at the moment is an increased police presence in public spaces,” De Maiziere told a news conference in Berlin.